U.S. patent number 6,247,525 [Application Number 09/576,729] was granted by the patent office on 2001-06-19 for vibration induced atomizers.
This patent grant is currently assigned to Georgia Tech Research Corporation. Invention is credited to Ari Glezer, Marc K. Smith.
United States Patent |
6,247,525 |
Smith , et al. |
June 19, 2001 |
Vibration induced atomizers
Abstract
A preferred embodiment of an atomizing apparatus incorporates a
source of heat transfer fluid and an atomizing surface adapted to
receive a droplet of the heat transfer fluid thereon. A driver also
is provided which is configured to control a vibration of the
atomizing surface at a frequency less than ultrasonic so that the
atomizing surface forms a spray of atomized droplets from the
droplet of the heat transfer fluid. Preferably, the vibration is
configured to form, on the droplet, surface waves having a smaller
wavelength than a diameter of the droplet, thereby ejecting and
propelling the atomized droplets from the droplet. Methods also are
provided.
Inventors: |
Smith; Marc K. (Marietta,
GA), Glezer; Ari (Atlanta, GA) |
Assignee: |
Georgia Tech Research
Corporation (Atlanta, GA)
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Family
ID: |
26718124 |
Appl.
No.: |
09/576,729 |
Filed: |
May 23, 2000 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
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044114 |
Mar 19, 1998 |
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Current U.S.
Class: |
165/104.25;
165/104.23; 165/104.34; 165/109.1; 239/102.1; 239/102.2 |
Current CPC
Class: |
B05B
7/0012 (20130101); B05B 17/0607 (20130101); F28F
13/02 (20130101); F28F 13/10 (20130101) |
Current International
Class: |
B05B
17/04 (20060101); B05B 7/00 (20060101); B05B
17/06 (20060101); F28F 13/00 (20060101); F28F
13/02 (20060101); F28F 13/10 (20060101); F28D
015/00 () |
Field of
Search: |
;165/104.21,104.23,104.25,104.31,104.33,104.34,109.1
;239/102.2,102.1 ;174/15 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Other References
Physical Review E: Viscous Effects In Droplet--Ejecting Cappillary
Waves; C.L. Goodridge, et al.; 1997; pp. 472-475. .
Solid-State Sensor and Actuators Workshop; Micromachined
Acoustic-Wave Liquid Ejector; X. Zhu, etal.; Jun. 2, 1996-Jun. 6,
1996; pp. 280-282. .
Annual Review of Fluid Mechanics, vol. 22; Paramtrically Forced
Surface Waves; Jo Miles, et al.; 1990; pp. 143-163. .
Physical Review Letters; Threshold Dynamics of Singular
Gravity-Capillary Waves; C.L. Goodridge, et al.; Mar. 11, 1996; pp.
1824-1827..
|
Primary Examiner: Yeung; James C.
Assistant Examiner: McKinnon; Terrell
Attorney, Agent or Firm: Thomas, Kayden, Horstemeyer &
Risley LLP
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a Continuation-In-Part Application which is
based upon and claims priority to U.S. patent application Ser. No.
09/044,114, filed on Mar. 19, 1998 (incorporated by reference
herein in its entirety), which is based upon and claims priority to
U.S. Provisional Application Ser. No. 60/041,422, filed Mar. 20,
1997 (incorporated by reference herein in its entirety).
Claims
What is claimed is:
1. A method of transferring heat from a heated body, comprising the
steps of:
providing a chamber having a first wall and a second wall spaced
therefrom, the chamber containing a heat transfer fluid;
arranging at least a portion of the first wall in a heat transfer
relationship with the heated body, the heated body being located
externally of the chamber;
placing a discrete quantity of the heat transfer fluid into contact
with the second wall; and
vibrating the second wall at a frequency less than ultrasonic to
disintegrate the liquid droplets into smaller secondary
droplets.
2. The method of claim 1, further comprising the step of:
propelling the secondary droplets away from the second wall such
that at least some of the secondary droplets impact an interior of
the first wall and vaporize, thereby transferring heat from the
first wall.
3. The method of claim 1, further comprising the step of:
condensing the heat transfer fluid through heat transfer to the
second wall, wherein the heat transfer fluid condenses and forms
liquid droplets along an interior of the second wall.
4. The method of claim 1, further comprising the step of:
dispensing the discrete quantity of the heat transfer fluid onto
the atomizing surface.
5. The method of claim 1, further comprising the step of:
cooling the second wall of the chamber.
6. The method of claim 1, wherein the step of vibrating the second
wall comprises the step of:
vibrating the second wall to form, on the liquid droplets, surface
waves having a smaller wavelength than a diameter of the liquid
droplets.
7. The method of claim 1, wherein the step of vibrating the second
wall comprises the step of:
utilizing power of less than 1 Watt to vibrate the second wall.
8. The method of claim 2, wherein the heat transfer fluid is water,
and wherein the step of propelling the secondary droplets comprises
the step of:
imparting a velocity of at least 1 m/s to at least some of the
secondary droplets.
9. An atomizing apparatus comprising:
a source of heat transfer fluid;
a sealed chamber having a first wall, a second wall and at least
one side wall extending therebetween, said first wall having an
exterior surface and an interior surface, said exterior surface
being configured to engage a heated surface, the heated surface
being arranged externally of said sealed chamber, said interior
surface being arranged inside said sealed chamber, said first wall
being configured to conduct heat from the heated surface and
transfer at least a portion of the heat to said interior surface,
said first wall opposing said second wall, said second wall having
an exterior surface and an interior surface arranged inside said
sealed chamber, said interior surface of said second wall being a
cool surface relative to the heated surface and being adapted to
receive a droplet of said heat transfer fluid;
a driver configured to control a vibration of said interior surface
of said second wall at a frequency less than ultrasonic such that
said atomizing surface forms a spray of atomized droplets from said
droplet of said heat transfer fluid, the vibration being configured
to form, on said droplet, surface waves having a smaller wavelength
than a diameter of said droplet, thereby ejecting and propelling
said atomized droplets from said droplet.
10. The atomizing apparatus of claim 9, further comprising:
a dispenser in fluid communication with said source of heat
transfer fluid, said dispenser being configured to dispense a
droplet of said heat transfer fluid on said interior surface of
said second wall.
11. The atomizing apparatus of claim 9, further comprising:
a piezoelectric element engaging said second wall and electrically
communicating with said driver such that said piezoelectric element
vibrates said interior surface of said second wall in response to
said driver.
12. The atomizing apparatus of claim 9, wherein said exterior
surface of said second wall is configured to engage a cooling
device, said cooling device being configured to maintain said cool
surface at a temperature cooler than a temperature of said interior
surface of said first wall.
13. The atomizing apparatus of claim 9, wherein said at least one
side wall is insulated to prevent condensation of said heat
transfer fluid therealong.
14. An atomizing apparatus comprising:
a source of heat transfer fluid;
a sealed chamber having a first wall, a second wall and at least
one side wall extending therebetween, said first wall having an
exterior surface and an interior surface, said exterior surface
being configured to engage a heated surface, the heated surface
being arranged externally of said sealed chamber, said interior
surface being arranged inside said sealed chamber, said first wall
being configured to conduct heat from the heated surface and
transfer at least a portion of the heat to said interior surface,
said first wall opposing said second wall, said second wall having
an exterior surface and an interior surface arranged inside said
sealed chamber, said interior surface of said second wall being a
cool surface relative to the heated surface and being adapted to
receive a droplet of said heat transfer fluid; and
means for controlling a vibration of a droplet of said heat
transfer fluid received on said interior surface of said second
wall, the vibration of the droplet being at a frequency less than
ultrasonic such that a spray of atomized droplets is formed from
said droplet of said heat transfer fluid, the vibration being
configured to form, on said droplet,
surface waves having a smaller wavelength than a diameter of said
droplet, thereby ejecting and propelling said atomized droplets
from said droplet.
15. The atomizing apparatus of claim 14, further comprising:
a piezoelectric element engaging said interior surface of said
second wall and electrically communicating with said means for
controlling a vibration of a droplet such that said piezoelectric
element vibrates said interior surface of said second wall in
response to said means for controlling a vibration of a
droplet.
16. The atomizing apparatus of claim 14, wherein said exterior
surface of said second wall is configured to engage a cooling
device, said cooling device being configured to maintain said cool
surface at a temperature cooler than a temperature of said interior
surface of said first wall.
17. The atomizing apparatus of claim 14, wherein said at least one
side wall comprises means for preventing condensation of said heat
transfer fluid along said at least one side wall.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to vibration induced
atomizers and, in particular, to vibration induced droplet and
vapor atomizers that may be utilized in heat transfer applications,
among others.
2. Description of the Related Art
Atomizers are commonly used in a variety of processes and devices.
Atomizers, basically, are concerned with breaking up materials,
typically liquids, into very small droplets, or particles.
Designers of these devices have created a wide range of atomizing
apparatuses and methods. For example, some atomizers collide a
gaseous stream into a liquid stream to break the liquid stream into
"atomized" droplets. Ultrasonic atomizers are also common.
Ultrasonic atomizers utilize ultrasonic waves, typically in the
megahertz frequency range, to atomize a liquid by focusing the
ultrasonic waves on the free-surface of the liquid. In other
applications, the ultrasonic vibrations are used to force liquid
through an array of holes, each of the holes being on the order of
tens of microns in size, to create a spray of atomized droplets.
Additionally, other types of atomizers are well known in the art
and used in a variety of applications.
Prior art atomizers, however, typically require some type of fluid
piping and fluid supply to operate or use bulky ultrasonic
transducers. Indeed, most atomizers are designed to constantly
inject an atomized liquid into a system. An atomizer that does not
require such fluid input to the system, but that is self-contained,
may be very useful in many applications, such as in heat transfer
devices. Additionally, an atomizer that combines rapid (even near
instantaneous) atomization of a discrete fluid droplet will be
advantageous in a wide variety of applications. Heat transfer is
one potential application for such a new atomizer.
Thermal management is a critical technology for many of today's
high performance devices. Particularly, thermal management is
critical to high performance vehicles and engines as well as
vehicles used in a microgravity environment, such space vehicles,
satellites, and the like. In hypersonic flight, for example, the
leading edge of an airfoil is subjected to intense frictional
heating that can raise the temperature of the airfoil's skin to
over the melting point. In advanced turbine engines, blade and vane
cooling is critical to prevent melting, erosion, and/or structural
failure of turbine blades and vanes. In a microgravity environment,
spacecraft power plants are cooled properly for efficient
operation. Similarly, the living environment of a spacecraft must
be maintained within the proper temperature range. Sensitive
scientific instruments used in space, such as low temperature
charge coupled diode (CCD) imagers, are maintained at a constant
uniform temperature in order to work effectively.
In addition, there is an ever-increasing demand for power in space
missions, such as the Space Lab project. Increasing the size of
power plants aboard such spacecraft brings with it an even larger
thermal management problem associated with the waste heat generated
by the system. Thus, effective cooling techniques are necessary in
all of these applications.
One popular technique for thermal control in aerodynamic
applications is film cooling. In this technique, air is injected
from small holes in the surface of the object to be cooled to form
a thin film of air flowing on the surface. The air film cools the
surface and effectively insulates it from the high-temperature gas
flowing past it.
Another popular technique for thermal management in these various
applications is the use of a "heat pipe." These devices are often
used in microgravity and aerodynamic applications because they can
accommodate a wide range of operating temperatures, can transport
large amounts of heat, and can operate independently of gravity. In
addition, relatively high heat transfer rates can be achieved by
heat pipes, which is typical of a phase-change heat transfer
device.
Heat pipes are relatively simple devices. Conceptually, heat pipes
passively transfer heat from a heat source to a heat sink, where
the heat is dissipated. The heat pipe itself is a vacuum-tight
vessel, typically cylindrical in shape, that houses a working
fluid. The working fluid typically comprises methanol, ethanol,
water, or another similar fluid. The vessel also houses a wick
element spanning the length of the vessel. As heat is directed into
one end of the heat pipe, the working fluid vaporizes, creating a
pressure gradient along the length of the pipe. This pressure
gradient forces the vapor to flow along the pipe to the cooler end,
where the vapor condenses, giving up its latent heat of
vaporization. The working fluid is then absorbed by the wick
element and moved by capillary forces back to the heated end of the
heat pipe.
While heat pipes have many advantages, heat pipes also have
critical limitations. In aerodynamic applications, for example, the
heat pipes must be capable of operating in the high g-loads typical
of a maneuvering fighter aircraft. Regardless of the application,
however, a major limitation of heat pipes is that the amount of
heat transfer performed by these devices is strictly governed by
the liquid flow rate produced by the capillary pumping in the
wicking material of the heat pipe. Thus, there exists a need for
improved apparatuses and methods which address these and other
shortcomings of the prior art.
SUMMARY OF THE INVENTION
Briefly described, the present invention generally relates to
vibration induced atomizers. In a preferred embodiment, an
atomizing apparatus incorporates a source of heat transfer fluid
and an atomizing surface adapted to receive a droplet of the heat
transfer fluid thereon. A driver also is provided which is
configured to control a vibration of the atomizing surface at a
frequency less than ultrasonic so that the atomizing surface forms
a spray of atomized droplets from the droplet of the heat transfer
fluid. Preferably, the vibration is configured to form, on the
droplet, surface waves having a smaller wavelength than a diameter
of the droplet, thereby ejecting and propelling the atomized
droplets from the droplet.
In another embodiment, an atomizing apparatus incorporates a source
of heat transfer fluid and a means for controlling a vibration of a
droplet of the heat transfer fluid at a frequency less than
ultrasonic so that a spray of atomized droplets is formed from the
droplet of the heat transfer fluid.
Other embodiments may be construed as providing a method for
transferring heat from a heated body. In a preferred embodiment,
the method includes the steps of: providing a chamber having a
first wall and a second wall spaced therefrom, the chamber
containing a heat transfer fluid; arranging at least a portion of
the first wall in a heat transfer relationship with the heated
body, the heated body being located externally of the chamber;
placing a discrete quantity of the heat transfer fluid into contact
with the second wall; and vibrating the second wall at a frequency
less than ultrasonic to disintegrate the liquid droplets into
smaller secondary droplets. Preferably, the secondary droplets are
propelled away from the second wall by its vibration so that at
least some of the secondary droplets impact an interior of the
first wall and vaporize, thereby transferring heat from the first
wall.
Other features and advantages of the present invention will become
apparent to one with skill in the art upon examination of the
following drawings and detailed description. It is intended that
all such features and advantages be included herein within the
scope of the present invention, as defined in the appended
claims.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS
The accompanying drawings, which are incorporated herein, and form
a part of the specification, illustrate the preferred embodiments
of the present invention and, taken together with the description,
serve to illustrate and explain the principles of the present
invention. As such, the drawings are not necessarily drawn to
scale, emphasis instead being placed on clearly illustrating the
principles of the invention. In the drawings:
FIG. 1 depicts a schematic side view of a preferred embodiment of a
basic vibration induced droplet atomizer.
FIG. 2 depicts a schematic side view of a preferred embodiment of a
heat transfer cell.
FIG. 3 depicts the heat transfer cell of FIG. 2 where the liquid
droplets have shattered into smaller secondary droplets.
FIG. 4 depicts the heat transfer cell of FIG. 2 after the secondary
droplets have impacted a heated surface of the cell chamber.
FIG. 5 depicts a schematic side view of an alternative embodiment
of a heat transfer cell.
FIG. 6 depicts the heat transfer cell of FIG. 5 where the vapor
bubbles have been shattered into smaller vapor bubbles.
FIG. 7 depicts the heat transfer cell of FIG. 5 where the smaller
vapor bubbles are circulated throughout the cell chamber.
FIG. 8 depicts a schematic side view of an alternative embodiment
of a heat transfer cell.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring now to the drawings, wherein like reference numerals
designate corresponding parts throughout the several views, a
preferred embodiment of a vibration induced droplet atomizer and
two preferred embodiments of heat transfer cells using the
atomizers will be described. As described in detail hereinafter, a
vibration-induced droplet atomizer of the present invention
preferably incorporates a flexible membrane mounted rigidly about
its periphery. A thin layer of piezo-ceramic material is adhered to
the underside of the membrane and time-varying voltage with an
arbitrary amplitude and frequency is applied to the piezo-ceramic
causing it to expand and contract. This motion causes the membrane
to move vertically up and down in response to the applied voltage
and creates an atomization of liquid residing upon the membrane.
(It should be noted that this is not ultrasonic atomization because
the present invention operates at lower frequencies; the spray that
is created produces droplets that typically are an order of
magnitude larger than those of ultrasonic atomizers and with much
larger velocities.)
For instance, a centimeter-sized droplet of some arbitrary liquid,
e.g., water, is placed on the membrane, such as at the center of
the top surface of the membrane, by any suitable method. The
piezo-ceramic is then energized with a sinusoidal voltage and a
given time-varying amplitude with a frequency of hundreds to
thousands of Hertz. The membrane starts to move up and down
producing waves on the surface of the droplet. If the correct
frequency and amplitude are used, the surface waves will have a
much smaller wavelength than the original droplet diameter and they
will begin to eject a smaller droplet or droplets from each wave
crest on each upward stroke. If the amplitude is large enough, the
entire volume of the original droplet can be converted into the
smaller droplets within a fraction of a second. The process looks
like a bursting phenomena, thus, we also call this droplet
bursting.
At a frequency of about 1 kHz, the ejected droplet size is about
400 microns and droplets move away from the membrane at velocities
of several meters per second. Therefore, it is not necessary to
have an external method (e.g., a fan, an air jet, etc.) to
transport the droplets away from the atomization site to where they
are needed, e.g., for evaporation. To do this successfully, the
membrane is moving up and down at about 200 microns peak to peak.
This produces an acceleration of about 400 g's at the surface of
the membrane. The membrane used in one embodiment of the present
invention is a thin steel plate about 1 inch in diameter. The power
used to create this atomization is on the order of a fraction of a
watt. Thus, the atomizing transducer is small, lightweight, and
requires very little power to function properly. The droplet size
and velocity produced by this process are also ideal for spraying.
This process can successfully spray a thin layer of liquid onto a
hot surface and, thus, effectively cool the surface by evaporation.
This is the reason why the present invention is described
hereinafter in relation to a heat transfer cell, although various
other applications are contemplated, and are considered well within
the scope of the present invention.
A. The Vibration Induced Droplet Atomizer
FIG. 1 depicts a preferred embodiment of a vibration induced
atomizer 10. The atomizer 10 preferably incorporates a diaphragm 15
which includes an atomizing surface 11. The diaphragm 15 is
attached at each of its ends to supports 20a, 20b. The diaphragm 15
may be attached by devices such as rivets, bolts, screws, or any
other device for suitably securing the diaphragm 15. The particular
attachment means used, as well as the particular design of the
supports 20a, 20b, will depend largely on where the atomizer 10
will be used and/or mounted.
A first side 12 of the diaphragm 15 is affixed with a device
capable of creating an oscillation of the atomizing surface 11.
Preferably, the oscillation creating device incorporates an array
of piezoelectric actuators 13a-13c. These actuators 13a-13c are
attached to the diaphragm 15 with an adhesive, such as glue, or
other appropriate means. Further, the piezoelectric actuators
13a-13c are connected, via wiring 14, to a driver 16. The driver 16
may include a wave generator, microcomputer, or other controllable
voltage source. The atomizer 10 also incorporates a fluid source 17
with a dispenser 18. The source 17 and dispenser 18 may be
configured as a syringe, a fluid injector, or other device capable
of dispensing a measured fluid droplet 19 onto the atomizing
surface 11. A basic schematic of an injector 18 is depicted in FIG.
1.
In operation, the driver 16 causes the piezoelectric actuators
13a-13c to vibrate. The vibration of the actuators 13a-13c creates
normal oscillation of the atomizing surface 11. As the atomizing
surface 11 oscillates, the source 17 and dispenser 18 place a
metered fluid droplet 19 onto the atomizing surface 11. The size of
the droplet 19 is a matter of choice depending on the application
where the atomizer 10 is utilized.
Once the fluid droplet 19 comes in contact with the atomizing
surface 11, the oscillation of the surface 11 creates waves in the
droplet 19. If the frequency and amplitude of the atomizing surface
11 oscillation is tailored to a value corresponding to the resonant
frequency for the size of the droplet 19, then an instability of
the liquid-gas interface occurs due to disturbances at the
vibrational frequency of the atomizing surface 11. The instability
manifests itself as a set of nonlinear surface waves that rapidly
grow in amplitude with a time constant that is primarily affected
by the excitation amplitude and the surface tension at the
interface. When the wave amplitude is of the order of the drop
height, the droplet 19 breaks up and is completely drained into a
spray of smaller (between one and two orders of magnitude)
secondary droplets 21 that are directed away from the surface 11.
The spray velocity near the atomizing surface 11 appears to depend
on the vibrational energy of the primary droplet 19 prior to its
breakup.
The relationship between the proper amplitude and frequency of
vibration and the droplet size can be determined without undue
experimentation by one skilled in the art, with droplet size being
determined based upon the requirements of the particular
application. For example, it is known that a water droplet having a
planform diameter of approximately 5 mm will break apart when the
atomizing surface 11 is operated at a frequency of approximately
1000 Hz and an amplitude of less than 100 .mu.m. The resonant
frequency increases with diminishing droplet size. Thus, one with
ordinary skill in the art will be able to determine the appropriate
frequency for a desired droplet size.
As alternatives to the atomizer 10 depicted in FIG. 1, the source
17 and dispenser 18 may be provided in other embodiments. For
example, the droplets could be received into orifices in the
diaphragm 15. If this were the case, the preferred dispenser 18 may
incorporate a tube for draining fluid from the source 17 to the
orifice in the diaphragm 15. The flow of fluid through the tube
could be regulated such that discrete portions of fluid are
deposited into the orifices. Typically, the flow regulator is an
electronically controlled valve along the tubing.
Of course the "source" may include the environment in which the
atomizer operates and the "dispenser" may include a natural
phenomenon such as condensation or boiling. The preferred
applications described below use these types of "sources" and
"dispensers" for the basic atomizer 10 described above.
Applications for such an atomizer may include fuel atomization,
biomedical applications, dispersion of a liquid into another
liquid, heat transfer, or many other applications. A preferred
application for the atomizer 10 described above is in the
construction of heat transfer cells. This preferred application
will now be described in detail below.
B. Heat Transfer Cell Using A Vibration Induced Droplet
Atomizer
1. First Preferred Embodiment
FIG. 2 depicts a heat transfer cell 30 of a first preferred
embodiment of the present invention. This first preferred
embodiment 30 incorporates a chamber 31. This chamber 31 can be of
many different shapes, however, the preferred embodiment 30
includes a chamber 31 shaped as a cylinder, such as with a
rectangular cross-section, for example, although various other
configurations may be utilized. Preferably, the chamber is sealed,
although other embodiments may not be not so-limited.
A first wall 32 of the chamber 31 is preferably attached to a hot
surface or heat-producing body 33. Alternatively, this first wall
32 may be a part of the heat-producing body itself. Preferably,
this first, heated wall 32 is the wall forming a first end of the
cylindrical chamber 31. A second wall 34 of the chamber 31 is
attached to a cool surface or cooling device 36. The cooling device
36 may incorporate such items as a radiator, a fan or other heat
transfer device. The selection of a proper cooling device 36
depends on the particular environment in which the heat cell 30
will be used. The cool wall 34 is preferably the wall forming a
second end of the cylindrical chamber 31. In this way, the heated
wall 32 and the cool wall 34 directly oppose one another. Lateral
walls 37a, 37b of the chamber 31 connect the two opposing end walls
32, 34 and form the remainder of the chamber 31. Note that the
other two lateral walls forming this chamber 31 are not depicted in
FIG. 2.
The chamber 31 of the first preferred embodiment 30 is filled with
a fluid 38 in a gaseous phase. This gas 38 can be of any
appropriate type for heat transfer applications but, preferably,
the gas 38 comprises water vapor.
An array of piezoelectric disks 39a-39d are attached to an exterior
surface 35 of the second, cool wall 34 of the chamber 31. The
piezoelectric disks 39 may be attached by glue or other appropriate
means understood in the art. The piezoelectric disks 39 are
attached via wiring 41 to a driver 42. This driver 42 causes the
piezoelectric disks 39a-39d to vibrate at a specific frequency and
amplitude. The driver 42 may be of any appropriate type of voltage
generating device, but preferably the driver 42 is a wave generator
that can be controlled for voltage output. The driver 42 may
incorporate a computer, or other logic circuitry, capable of
voltage output to the piezoelectric disks 39a-39d. As the
piezoelectric disks 39a-39d are caused to vibrate by the driver 42,
the second end wall 34 moves in periodic motion normal to the
exterior surface 35 of the second wall 34.
Although not a requirement of the preferred embodiment of the
present invention, the second, cooled wall 34 of the chamber 31 may
be outfitted with specifically constructed condensation sites 46
aligned with the piezoelectric disks 39a-39d. Such sites 46 are
typically constructed as recesses on an interior surface 40 of the
second wall 34 of the chamber 31. As the temperature of the gas 38
rises, the gas 38 will begin to condense along the interior surface
40 of the cool wall 34 at the specifically constructed condensation
sites 46. As a result, condensation droplets 43a, 43b, form along
the surface 40 and begin to grow.
In some applications, it may not be desirable that the gas 38
condenses along the lateral walls 37a, 37b of the chamber 31. To
this end, the lateral walls 37a, 37b can be insulated, or even
slightly heated, in order to prevent condensation along the
interior surfaces of these walls 37a, 37b. However, in other
applications, the gas may be allowed to condense along the lateral
walls, whereby the condensate may merely be gravity fed down the
walls and to the surface 40.
The response of the liquid droplets 43a, 43b to the normally
vibrating second end wall 34 is initially no more than solid-body
vibration along with the second wall 34. Through the natural
process of condensation along the cool interior surface 40 of the
second end wall 34, the liquid droplets 43a, 43b begin to grow in
size. When these droplets 43 reach a critical size, the free
surface instability produced by the vibration of the piezoelectric
disks 39a-39d causes the droplets 43 to produce waves. If the
amplitude of the oscillation of the wall 34 is large enough, the
droplets 43 will disintegrate into a spray of smaller, secondary
droplets 44, as depicted in FIG. 3. The secondary droplets 44 are
propelled away from the cool interior surface 40 of the second wall
34 and across the chamber 31.
As depicted in FIG. 4, the secondary liquid droplets 44 impact the
chamber wall opposite to the second end wall 34, the heated
surface, or first end wall 32. Upon impact, these droplets 44
spread out and are vaporized. This evaporation process transfers
heat from the first heated end wall 32 into the vapor 38. The
evaporation of the droplets 44 produces a large vapor pressure in
the vicinity of the heated first end wall 32. This increased vapor
pressure forces the vapor 38 away from the first end 32 of the
chamber 31 and toward the cool end wall 34 of the chamber 31. As
outlined above, as the vapor contacts the cool interior surface 40
of the second end wall 34, the vapor 38 condenses to form the
liquid condensate droplets 43 used to create the spray of secondary
droplets 44. Thus, the cycle will continue to transfer heat away
from the heated first end wall 32 to the second end wall 34 of the
first preferred embodiment 30. If the liquid droplets 43 are
continually replaced by condensing gas, then the spray of secondary
droplets 44 will be nearly continuous.
2. Second Preferred Embodiment
A second preferred embodiment 50 of a heat transfer device using a
vibration induced atomizer of the present invention is depicted in
FIG. 5. The second preferred embodiment 50 generally includes a
heat transfer cell based on nucleate boiling technology implemented
with a vibration induced atomizer. The present embodiment of heat
transfer cell 50 incorporates a chamber 51 with walls. Although
many different shapes of chambers may be used with the second
preferred embodiment 50, a cylindrical chamber 51 with a
rectangular cross-section has been selected. As such, the chamber
is defined by a first end wall 53 and a second end wall 56 directly
opposing this first end wall 53. The chamber also has four lateral
walls 66a, 66b (only two lateral walls are depicted) connecting the
first and second end walls 53, 56.
The chamber 51 of the second preferred embodiment 50 is preferably
sealed from an outside environment 52; however, a sealed chamber is
not required. The entire chamber 51, whether sealed or not, is
filled with a working fluid 61 principally in a liquid phase. This
fluid may include fluids such as water, methanol, ethanol, or
refrigerants. The present invention is not limited to the use of
any particular fluid, although water is the preferred heat transfer
liquid.
The first end wall 53 of chamber 51 is attached to a heat-producing
body or surface 54. Alternatively, first end wall 53 could be
merely placed directly adjacent to the heated body (or device) 54,
or the end wall 53 could incorporate the heated itself. This first
end wall 53 is preferably one of the end walls of the cylindrical
chamber 51. As mentioned above, a second end wall 56 directly
opposes the first end wall 53. This second wall 56 is preferably
connected to a cooled surface or cooling device 57. As above, the
cooling device 57 may include such items as a radiator, fan or
other heat transfer device. The selection of a proper cooling
device 57 depends on the particular environment in which the heat
cell 50 will be used.
Interior to the chamber 51, there are preferably a series of heat
exchange surfaces or fins 58a-58c. These heat exchange fins 58a-58c
are preferably connected to the second end wall 56 and cooled
thereby. A typical arrangement of these fins 58a-58c is depicted
FIG. 5; although other arrangements of fins 58a-58c are
contemplated. The goal in arranging fins 58a-58c is usually to
permit circulation of the fluid 61 throughout the chamber 51, while
exposing a great amount of surface area to the working fluid 61.
Although fins 58a-58c are not necessary, these fins 58a-58c provide
increased surface area for heat exchange and a generally more
efficient heat transfer cell 50.
On an exterior surface 55 of the first end wall 53, there are
preferably attached an array of piezoelectric disks or elements
62a-62d. These piezoelectric elements 62 can be attached by glue or
any other appropriate adhesive. The piezoelectric array 62 is
connected by wiring 63 to a driver 64. The driver 64 drives the
piezoelectric disks 62 such that the first wall 53 is vibrated at a
given frequency and amplitude and caused to oscillate normal to its
surface 55. The driver 64 may incorporate any
controlled/controllable source of voltage, such as a generator or
computer.
Although not required by the preferred embodiment 50, the lateral
walls 66a, 66b may be insulated. This improvement may improve the
performance of the heat transfer cell in certain applications.
As the first wall 53 begins to heat up, heat is transferred to the
liquid 61 adjacent to an interior surface 60 of the first end wall
53. Eventually the liquid 61 will begin to boil. Boiling produces
vapor bubbles 67a, 67b attached to the interior surface 60 of the
first end wall 53. These vapor bubbles 67a, 67b increase in size as
the temperature of the liquid 61 increases and boiling
continues.
As the boiling liquid may alter the pressure of the liquid 61 in
the chamber, it is desirable that a primary chamber 51 be connected
through a series of fluidic piping 68 to an auxiliary chamber 69
where reserve fluid may be stored in order to keep the pressure
inside the primary chamber 51 equal. The flow of fluid between the
chamber 51 and the reserve chamber 69 is typically controlled by a
computer-operated valve 71. Of course, other logic circuitry will
function equally well to a computer control system 72. The control
system 72 will preferably receive pressure data on the interior
pressure of the primary chamber 51 from a pressure sensor 73. As
the pressure changes in the chamber 51, the control system 72
alters the flow of fluid through the valve 71 to keep the pressure
in the chamber 51 at a pre-selected value.
Along one of the lateral walls 66b of the second preferred
embodiment 50, there is positioned a synthetic jet actuator 74.
Generally, a synthetic jet actuator incorporates a housing defining
an internal chamber. An orifice, or opening, is defined by a wall
of the housing. The synthetic jet actuator further includes a
mechanism in or about the housing for periodically changing the
volume within the internal chamber. As the volume of the synthetic
jet chamber is decreased, a series of fluid vortices are generated
at the orifice and projected into the chamber. These vortices move
away from the edges of the orifice under their own self-induced
velocity and synthesize a jet of fluid through entrainment of the
chamber liquid 61. As the volume of the synthetic jet chamber is
increased, fluid 61 is drawn from the orifice into the synthetic
jet chamber. Since the vortices are already removed from the edges
of the orifice, they are not affected by the fluid 61 being
entrained into the synthetic jet chamber. In operation, the
synthetic jet actuator creates a jet of fluid without creating any
net mass change in the heat cell chamber 51.
Synthetic jet actuators are fully described in, among others,
copending patent application No. 08/489,490, filed Jun. 12, 1995.
This application is hereby incorporated by reference as if fully
set forth herein. The synthetic jet actuator 74 used in the present
invention creates a fluid flow (or current), depicted by arrow 76,
across the heated wall 53 of the chamber 51.
As mentioned above, as the heat transfer to the liquid 61
increases, the vapor bubbles 67 continue to grow in size. When the
vapor bubbles 67 reach a critical size related to the vibration
frequency of the piezoelectric disks 62, the free-surface
instability produced by the vibration will produce waves on the
vapor bubbles and, for large enough vibration amplitudes, generate
a cloud of smaller, secondary bubbles 77 from the vapor bubbles 67.
The larger vapor bubbles 67a, 67b are usually not completely
disintegrated into the secondary bubbles 77 and are typically still
in contact but are released from the grip of contact-angle
hysteresis with the interior surface 60 of the first end wall 53.
See FIG. 6. The synthetic jet 74 not only creates a flow 76 of
fluid, or current, across the interior surface 60 of the first wall
53, but this flow 76 circulates throughout the chamber 51 such that
the fluid 61 is exposed to all the surfaces of the fins 58. The
flow of the fluid is depicted in FIG. 6 by the arrows 78a-78c.
A unique characteristic of the synthetic jet 74 is the very strong
entrainment of fluid 61 into its flow 76. As such, the flow 76 will
entrain both the tiny vapor bubbles 77 and the larger vapor bubbles
67. The flow 76 will carry these bubbles 67, 77 away from the
interior surface 60 of the first end wall 53. Because of the strong
entrainment by the jet 74, the working fluid 61 with the bubbles
67, 77, will be circulated through the cooled conducting partitions
or fins 58 attached to the cold surface 56 in order to improve the
transfer performance of the cell 51. See FIG. 7. At the fins 58, or
at the cooled surface 56, the bubbles 67, 77 will condense back
into a liquid phase and complete the heat transfer cycle in the
cell 50.
A modification of the second preferred embodiment 80 is depicted in
FIG. 8. This modification 80 includes a chamber 51 with walls. As
above, the chamber 51 incorporates a first end wall 53 and a second
end wall 56 directly opposing this first end wall 53. The chamber
also includes four lateral walls 66a, 66b (only two depicted)
connecting the first and second end walls 53, 56. As described
above, the chamber 51 is filled with a heat transfer liquid 61. The
first end wall 53 of this chamber 51 is attached to a
heat-producing body or heated surface 54. The second wall 56 is
preferably connected to a cooling device or cooled surface 57.
On an exterior surface 55 of the first end wall 53, there is
preferably attached an array of piezoelectric disks or elements
62a-62d. The driver 64 drives the piezoelectric disks 62 such that
the first wall 53 is vibrated at a given frequency and amplitude
and caused to oscillate normal to its surface 55.
As the first wall 53 begins to heat up, heat is transferred to the
liquid 61 adjacent to the interior surface 60 of the first end wall
53. Eventually, the liquid 61 will begin to boil. Boiling produces
vapor bubbles 67a and 67b attached to the interior surface 60 of
the first end wall 53. These vapor bubbles 67a, 67b increase in
size as the temperature of the liquid 61 increases and boiling
continues.
When the vapor bubbles 67 reach a critical size related to the
vibration frequency of the piezoelectric disks 62, the free-surface
instability produced by the vibration will produce waves on the
vapor bubbles and, for large enough vibration amplitudes, generate
a cloud of smaller, secondary bubbles 77 and release the larger
vapor bubbles 67a, 67b from the grip of contact-angle hysteresis on
the interior surface 60 of the first end wall 53.
A synthetic jet actuator 81 is located at the center of the cell
chamber 82 and attached to a heat sink fin 83. The heat exchange
fins 83, 86a-86d are preferably connected to the second end wall
56. These fins 83, 86a-86d permit circulation of the fluid
throughout the chamber 51, while exposing a great amount of surface
area to the working fluid 61.
The synthetic jet actuator 81 is driven such that a fluid jet 84
will time-periodically sweep across the heated surface 60, thus
providing a localized momentary stagnation point flow which may
improve the performance of the cell 80 in certain applications. As
above, the synthetic jet actuator creates a flow 84 of fluid that
circulates throughout the chamber 51 such that the fluid 61 is
exposed to all the surfaces of the fins 83, 86a-86d. The flow of
the fluid is depicted in FIG. 8 by the arrows 78a-78d.
As described above, the synthetic jet flow 84 will entrain both the
tiny vapor 77 and the larger vapor bubbles 67. The flow 84 will
carry these bubbles 67, 77 from the interior surface 60 of the
first end wall 53. Because of the strong entrainment by the jet 84,
the working fluid 61 with the bubbles 67, 77, will be circulated
through the cooled conducting partitions or fins 83, 86a-86d
attached to the cold surface 56 in order to improve the heat
transfer performance of the cell 51. Near the fins 83, 86a-86d, or
near the cooled surface 56, the bubbles 67, 77 will condense back
into a liquid phase and complete the heat transfer cycle in the
cell 80.
The foregoing description has been presented for purposes of
illustration and description. It is not intended to be exhaustive
or to limit the invention to the precise forms disclosed. Obvious
modifications or variations are possible in light of the above
teachings. The embodiment or embodiments discussed, however, were
chosen and described to provide the best illustration of the
principles of the invention and its practical application to
thereby enable one of ordinary skill in the art to utilize the
invention in various embodiments and with various modifications as
are suited to the particular use contemplated. All such
modifications and variations, are within the scope of the invention
as determined by the appended claims when interpreted in accordance
with the breadth to which they are fairly and legally entitled.
* * * * *